The surface microlayer (Surface Microlayer, SML) of a water body is a thin layer between the air and the water body, and is the only road for exchange of substances in the air-water interface. That is, from a microscopic aspect, almost all substances enter or leave the water body through a boundary of the water body surface microlayer for air-water exchange and convert in the surface microlayer. The water surface microlayer has special physical-chemical-biological properties, and has direct and important influences on the biogeochemical cycle of the water body, the air-water exchange flux of substances and even the climate, etc.
The composition and property of the water body surface microlayer are associated with different samplers and the thickness of the surface layer that is taken. Normal samplers include various sieve and rotary drum type surface layer samplers; moreover, the surface layer samples may also be collected through a manner of vertically getting water from the water using a glass plate. The methods above are generally called as active methods, and the thicknesses of the surface layers obtained are around 100 microns mostly. Wherein, the sieve and glass plate sampling manners are suitable for rivers, lakes and offshore regions; because the concentration of the contaminants is low and the detection line is high, a relatively large amount of sample volume is required, so that a large number of labour power and time are required. The rotary drum surface layer sampler can save the labour power and time, but the cost thereof is high and the volume thereof is relatively large; therefore, it is suitable for correlational studies in ocean regions.
However, the results obtained through the active method are instantaneous concentrations mostly (short time concentration), and related results of geochemical cycle and air-water exchange flux in a relatively long time period cannot be obtained, which is not beneficial for developing studies on an air-water heterogeneous interface (phase transformation). Meanwhile, the dissolved concentration of the water body obtained needs to be corrected, so as to obtain the freely dissolved concentration.
Sampling solutions in the correlational studies about the air-water exchange flux include (1) respectively sampling one point in the air and one point in the water body to conduct studies about exchange of substance and flux; however, in this method, there is no uniform specification for sampling height (defined by the sampling condition) in the academic circle; and (2) setting a plurality of sampling heights in the atmospheric layer to fit the results. This method infers atmospheric precipitation or water body volatilization according to the concentration changes of each point; however, in this method, the influences of the water body surface microlayer on the air-water body exchange are not taken into consideration, and the air-water body exchange may be overrated/underrated.